1. Field of the Invention
The present invention relates to a method of producing electrical resonant circuits, specifically resonance labels, for use in a theft protection system in which a coil and a capacitor located on a dielectric form a respective resonant circuit, where the coil having the shape of a spiral and one surface of the capacitor are located at the one side and the second surface of the capacitor is located at the other side of the dielectric.
2. Description of the Prior Art
Electrical resonant circuits of this kind placed onto a supporting foil and thereafter treated further in order to be used as adhesive labels or hard shells of a plastic material in a theft protection system have until now been produced predominantly by means of an etching method. Thus two aluminum foils are separated from each other by a non-conductive layer such as a plastic material, in which by means of printing of an etching medium resistant lacquer onto the two aluminum foils the desired shape of the resonant circuit consisting of a coil and a capacitor is formed. When the undesired parts of the aluminum foil are removed by the etching procedure a coil in shape of a planar spiral and a first capacitor surface remain on the one side of the supporting foil and a second capacitor surface is produced by the etching procedure on the other side of the supporting foil, which surface still is to be electrically connected to the first one.
When using a resonance label equipped with such a resonant circuit in a theft protection system the resonant circuit is deactivated or switched off upon paying for the wares equipped with such a label at the cash register which proceeds by a spark discharge or burn out of the capacitor. If the ware has not been paid for, a current flow is induced in the resonance circuit when passing through an electromagnetic field at the exit of the store which triggers an alarm.
In order to make the burn out of the capacitor for the deactivating possible, it previously had been the practice to thin out the dielectric between the two capacitor surfaces at a certain area whereby a thinning of the dielectric down to 1-3 microns is necessary, because otherwise no electric discharge between the two surfaces of the capacitor can be achieved because the level of the current which may be used is limited by regulations. Due to measurements which have been made it is known that for a thickness of 25 microns of a dielectric consisting for instance of polyethylene, about 500 Volts are needed. In case of a thickness of 1-3 microns achieved by a thinner dielectric, 5-10 Volts are still necessary for the discharge. The thickness of the dielectric of 1-3 micron must be carefully maintained because otherwise in dimension contacting of the aluminum surfaces of the capacitor can occur such that the resonance circuit is shorted out prior to its use. Also, in case of an extremely thin layer, the magnetic field in the detection system may deactivate the resonance label. If, at the other hand, the thickness of the dielectric is too high, the current level generated by the deactivating station at the cash register cannot produce the burn out of the capacitor, such that a deactivating is not possible.
Upon the electrical discharge a fine thread of deposited aluminum vapor is produced which is quite brittle. If the thinning of the dielectric matter by means of tools and by an application of pressure and/or temperature has not been made extremely carefully, it is possible that cracks or grooves develop in the dielectric at the thinned area or the material of the dielectric is partly displaced. In such case the thread of aluminum produced during the deactivation remains no longer embedded and supported in the material of the dielectric and rather hangs exposed in a free space between the two aluminum surfaces and will easily break with slight vibration and bending such that the resonance label is reactivated. The result of this is that the client, although he has paid for his purchase at the cash register triggers an alarm when passing the safety system at the exit of the store and is stopped erroneously which leads to considerable inconvenience.